Adsorption/desorption reactions between polar gases and certain metal salts yield complex compounds which are the basis for efficient refrigeration, thermal storage and heat pump systems having high energy density. However, energy density, a measure of the quantity of polar gas adsorbed on the salt, which translates into the amount of work or energy which can be stored in a given amount of the complex compound, is only one parameter to be considered in designing commercially attractive systems. Of significance, if not greater importance, are the reaction rates between the gas and the complex compound, which result in the time it takes to adsorb and desorb a given amount of the gas from the complex compound. Increased or maximized reaction rates result in increased or improved power that can be delivered by the system, i.e., more energy delivered over a period of time, which translates into greater heating and/or cooling capability of the system. In the aforesaid application, there is described a method and apparatus for achieving high reaction rates in solid-gas reactor systems and improve heat and mass transfer, i.e., thermal conductivity and gas diffusion in the reacting mass, by controlling the desirable density of the solid reactant during adsorption of a gaseous reactant. A preferred method disclosed for improving the reaction rates comprises restricting the volumetric expansion of the complex compounds formed between polar gaseous reactants and solid metallic salts during the chemisorption reactions. The results of such methods yield complex compounds having physical properties further disclosed herein.